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Tag: modernized NSRS

  • The effects of tectonic plate movement on the modernized 2022 NSRS

    The effects of tectonic plate movement on the modernized 2022 NSRS

    It’s the beginning of 2022 and the new, modernized NSRS is only about three years away. Hopefully, everyone has been reading NGS’s blueprint documents updated during 2021, and participating in NGS’s webinar series. Together, they provide the latest information about the changes from the existing NSRS to the new NSRS.

    My previous columns highlighted many aspects of the new geometric reference frame and geopotential datum. In this month’s column, I will highlight the time-dependent aspect of the modernized NSRS and why it is necessary for the new system.

    As I stated before, NOAA’s National Geodetic Survey (NGS) is developing models and tools for users to be able to transform coordinates between the four national terrestrial reference frames and the International Terrestrial Reference Frame, the Geopotential Datum and the North American Vertical Datum of 1988 (NAVD 88), as well as estimate coordinates at epochs different from the survey observation epoch by accounting for movement.

    What does NGS mean by estimate coordinates at epochs different from the survey epoch, and why is it necessary to account for movement for the new, modernized NSRS? This column will address these issues.

    NGS’s January 2022 (Issue 27) edition of NSRS Modernization News announced a paper about the modernized NSRS and a change in name to the Intra-Frame Velocity Model (IFVM). See the box below. Users can sign up for these newsletters here,  and can obtain access to previous newsletters here.

    The Latest Issue of
    NSRS Modernization News

    Image: From GovDelivery Communications Cloud on behalf of: NOAA's National Ocean Service)
    Image from GovDelivery Communications Cloud on behalf of NOAA’s National Ocean Service.

    The new paper was published in October 2021 and is titled “The Mathematical Relation between IFVM2022 as Expressed in ITRF2020 with IFVM2022 as Expressed in the Four Terrestrial Reference Frames of the Modernized NSRS with Dependence on EPP2022.” It can be downloaded here.

    The paper describes the mathematical relationship between the Intra-Frame Velocity Model (IFVM2022) and the Euler Pole Parameters (EPP2022).

    The NSRS Modernization News announcement states that the IFVM2022 name has been changed to the Intra-Frame Deformation Model (IFDM2022). The latest version of blueprint 1 and the October 2021 (NOS NGS 90) report were published before the name changes, so they refer to IFVM2022 instead of IFDM2022.

    Photo:

    Why is it necessary to account for movement? Coordinates basically change because the Earth’s surface is moving due to the movement of major tectonic plates. See the box below for information about why it is called plate movement or tectonic shift. NGS understands this and is attempting to manage the changing coordinates by providing a time-dependent component.

    Image: National Ocean Service Website
    Image: National Ocean Service website
    Screenshot: NOAA Website
    Screenshot: NOAA Website

    NGS will be defining the following four geometric terrestrial reference frames that are based on the tectonic plates (see map below):

    • North American Terrestrial Reference Frame of 2022 (NATRF2022)
    • Pacific Terrestrial Reference Frame of 2022 (PATRF2022)
    • Caribbean Terrestrial Reference Frame of 2022 (CATRF2022)
    • Mariana Terrestrial Reference Frame of 2022 (MATRF2022)

    Four Tectonic Plates Part of NGS’s New NSRS

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    As previously stated, NGS is developing models and tools for users to be able to transform coordinates between the four national frames and the International Terrestrial Reference Frame, as well as estimate coordinates at epochs different from the survey observation epoch by accounting for movement. These models are denoted as EPP2022 and IFDM2022.

    So, what are EPP2022 and IFDM2022? And what does this mean to surveyors and mappers?

    EPP stands for Euler pole parameters (a way of describing a plate’s rotation) and IFDM2022 is a way of computing the drift in coordinates.

    Why Euler Pole? See the box titled “Who was Euler?”

    Who was Euler?

    Leonhard Euler was a Swiss who lived in the 1700s. He was one of the greatest mathematicians that ever lived and has been called the greatest mathematician of the 18th century. He founded the studies of graph theory and topology, and made pioneering and influential discoveries in many other branches of mathematics such as infinitesimal calculus. He introduced a lot of modern mathematical terminology and notation, including the notion of a mathematical function. He is also known for his work in mechanics, fluid dynamics, optics, astronomy and music theory.

    The definition of Euler’s fixed point theorem states that any motion of a rigid body on the surface of a sphere may be represented as a rotation about an appropriately chosen rotation pole, called a Euler pole. This theorem has been used by geologists to understand and describe the motions of tectonic plates.

    NGS’s 2021 revised Blueprint 1, NOAA Technical Report NOS NGS 62, Blueprint for the Modernized NSRS, Part 1: Geometric Coordinates and Terrestrial Reference Frames provides an explanation of Euler poles and “plate-fixed” frames. As stated in the “Who was Euler?” box, the definition of Euler’s fixed-point theorem states that any motion of a rigid body on the surface of a sphere may be represented as a rotation about an appropriately chosen rotation pole, called a Euler pole. The following is stated in the NOS NGS 62 report under “Plate-Fixed Frames and Euler Poles,” section 4:

    When considering only the rigid (not deforming) part of a tectonic plate, the horizontal motion of the plate (relative to a global plate-independent reference frame, like the ITRF) can be modeled as a rotation about a geocentric axis passing through a fixed point on Earth’s surface. Although such models must make certain assumptions (such as the rigidity of the plate), the dominant motion of the majority of points on most tectonic plates is the rotation about a fixed point. That point is known as an “Euler pole.”

    What is important to know is that the determination of a plate’s Euler pole location and the angular velocity with which the plate rotates can be empirically determined using GNSS observations from a CORS network distributed throughout the plate. Figure 1 from the NOS NGS 62 report provides a plot of the North American plate Euler pole and the vectors of the horizontal velocities at select CORS (see the box titled “Figure 1 from NOS NGS 62”).

    Figure 1 from NOS NGS 62

    Photo: NGS Website
    Photo: NGS website

    Every place on Earth is moving. That includes neighboring marks on the same tectonic plate. What this means is that after the Eulerian motions are removed, the remaining motions left over change the relative differences in coordinates of neighboring marks located on the same tectonic plate. Figures 2 and 3 from the NOS NGS 62 report provide plots of estimates of these remaining velocities (see the boxes titled “Figure 2 from NOS NGS 62” and “Figure 3 from NOS NGS 62.”)

    Figure 2 is a plot of the non-Eulerian motions east of 110° west longitudes. As stated in the report, most of the velocities are less than 2 mm/year. The concept is that the EPP2022 and IVDM2022 models will remove the Eulerian and non-Eulerian movement of the marks.

    Figure 2 from NOS NGS 62

    Image: NGS Website
    Image: NGS website

    Figure 3 is a plot of non-Eulerian vectors west of 110° west longitude. As indicated in the plot, the large vectors in Western California, Western Oregon and Western Washington show areas of deformation near plate boundaries that don’t appear to be adequately captured just from the North American plate rotation.

    Figure 3 from NOS NGS 62

    Image: NGS Website
    Image: NGS website

    It should be noted that the size of the vectors on Figures 2 and 3 depict a different magnitude of movement. Figure 2 depicts vectors at 1-3 mm/year and Figure 3 depicts movement at 10-30 mm/year.

    To better visualize the potential size of the movement, I downloaded the CORS ITRF2014 coordinates and velocities from NGS’s website and compiled the results. See the boxes titled “CORS ITRF 2014 Horizontal Velocities” and “Table of ITRF 2014 Horizontal and Upward Velocities of U.S. CORSs.”

    CORS ITRF 2014 Horizontal Velocities

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    The box titled “CORS ITRF 2014 Horizontal Velocities” provides the horizontal vectors based on NGS’s file downloaded on Jan.13. Only CORSs designated as operational and computed velocities were included in the plot.

    I have also created a table that includes a summary of the ITRF rates for CORS labeled as part of the United States. The table includes the following information for each State and Territory of the United States:

    1. Number of CORS
    2. Minimum Horizontal Velocity (mm/year)
    3. Maximum Horizontal Velocity (mm/year)
    4. Average Horizontal Velocity (mm/year)
    5. Minimum Upward Velocity (mm/year
    6. Maximum Upward Velocity (mm/year),
    7. Average Upward Velocity (mm/year).

    See the table below.

    Table of ITRF 2014 Horizontal and Upward Velocities of U.S. CORSs

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Highlighted Territories are not on the North American Plate (GU, HI, PR, and VQ) and higlighted States are partly inside or close to the boundary of the North American Plate and another tectonic plate (AK, CA, OR, WA).
    Highlighted territories are not on the North American plate (GU, HI, PR, and VQ), and highlighted states are partly inside or close to the boundary of the North American plate and another tectonic plate (AK, CA, OR, WA).

    The highlighted territories in the table are not on the North American plate (GU, HI, PR and VQ), and the highlighted states are partly inside or close to the boundary of the North American plate (CA, OR, WA). This is one of the reasons why their minimum and maximum horizontal velocity values are different from most of the other states’ values.

    To visualize the relative differences in horizontal velocities between neighboring CORSs, I plotted the ITRF 2014 Horizontal Velocities for CORSs located in North Carolina (see the box titled “CORS ITRF 2014 Horizontal Velocities in North Carolina”). Looking at the figure, it’s obvious that all of the velocities are around 14 mm/year and moving in the same direction.

    CORS ITRF 2014 Horizontal Velocities in North Carolina

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Photo: Dave Zilkoski
    Screenshot: Dave Zilkoski

    I plotted the horizontal velocities for Missouri to provide an example of the velocities in the central region of the conterminous United States. The magnitude of the velocities is similar to that for North Carolina, but the direction of the vector is slightly different. North Carolina’s average horizontal velocity is 14.1 mm/year and Missouri’s average horizontal velocity is 14.6 mm/year.

    CORS ITRF 2014 Horizontal Velocities in Missouri

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    To emphasize the differences along the boundaries of the tectonic plates, I’ve included a plot of the CORS ITRF 2014 horizontal velocities for the State of Oregon and a plot of the states along the West Coast of the United States. See the boxes titled “CORS ITRF 2014 Horizontal Velocities in Oregon” and “CORS ITRF 2014 Horizontal Velocities Along West Coast of CONUS.” As indicated in the plot, there are significant changes in horizontal velocities near the Oregon coast. The values decreased by about 10 mm/year from the inland CORS to the CORS along the coast.

    CORS ITRF 2014 Horizontal Velocities in Oregon

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Image: David Zilkoski
    Image: Dave Zilkoski

    The plot of the CORS ITRF 2014 Horizontal Velocities Along West Coast of CONUS clearly indicates the change in magnitude the closer the CORS are to the Pacific and Juan de Fuca plates.

    CORS ITRF 2014 Horizontal Velocities Along West Coast of CONUS

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Image: David Zilkoski
    Image: Dave Zilkoski

    For completeness, I’ve also included a plot of the horizontal velocities for Alaska.

    CORS ITRF 2014 Horizontal Velocities in Alaska

    Computed Velocities Only (Downloaded Jan. 13, 2022)

    Image: David Zilkoski
    Image: Dave Zilkoski

    To better visualize the horizontal and upward velocities of CORS among states, I plotted the average horizontal and upward velocity value for each state based on that states’ CORS. See the box titled “Average Velocities by State.”

    Average Velocities by State

    Image: David Zilkoski
    Image: Dave Zilkoski

    I also computed an average horizontal velocity value based on CONUS CORS east of 110° west longitude (denoted here as a regional horizontal velocity value). [I used the CORSs east of 110° west longitude to be consistent with NGS’s Figure 2 in NOS NGS 62.]

    The box below summarizes the average horizontal motion for each state. The table provides:

    1. The Number of CORS East of 110° West Longitude
    2. Average Horizontal Velocity (mm/year)
    3. Average Horizontal Velocity minus Regional Horizontal Velocity (mm/year).

    This provides an estimate of the variation of the relative horizontal motion between States.

    Table of ITRF 2014 Horizontal Velocities minus Regional Velocity of U.S. CORS East of 110° West Longitude

    Image: David Zilkoski
    Table only includes CORS East of 110° West Longitude (Image: Dave Zilkoski)

    The box titled “Horizontal Velocities in NC Minus Average Velocity” depicts the resulting horizontal velocities with an average velocity removed (the average velocity was based on NC CORS only) for all CORS in North Carolina. As one can see from the plot, most of the resulting horizontal velocities are less than 1 mm/year, but they are still not zero. Once again, this is only meant to provide an idea of the size of the relative vectors between CORS in North Carolina.

    As indicated in the NOS NGS 62 report, these horizontal velocities will be small, but they will not be zero. Hence the reason that NGS needs to provide models and tools for users to be able to transform coordinates between the four national frames (NATRF, PATRF, CATRF and MATRF) and the International Terrestrial Reference Frame (ITRF), as well as to estimate coordinates at epochs different from the survey observation epoch by accounting for movement within the reference frame. Surveyors in California have been dealing with these types of movements for many years now.

    Horizontal Velocities in NC Minus Average Velocity

    (Downloaded Jan. 13, 2022)

    Image: David Zilkoski
    Image: Dave Zilkoski

    I plotted the ITRF 2014 upward velocity values of the CORS in North Carolina to depict an estimate of the vertical movement of the CORS in North Carolina. See the box below. The vertical velocities values are much less than the horizontal velocities, but they still are not zero. A future column will address the upward velocities based on the ITRF 2014 rates and crustal movement models.

    CORS ITRF 2014 Upward Velocities in North Carolina

    (Downloaded Jan. 13, 2022)

    Image: Dave Zilkoski
    Image: Dave Zilkoski

    This column explained why it is important to account for movement of marks everywhere and not just in areas influenced by active crustal movement due to earthquakes such as in Southern California. It provided information about the CORS rates of movement based on NGS’s ITRF2014 coordinates and velocity information. It highlighted NGS’s reports that describe models that will facilitate users transferring coordinates between reference frames and dealing with intra-frame movement between marks based on survey performed at different epochs. This is not just a horizontal positioning issue.

    A future column will address estimates of vertical velocities in the new, modernized NSRS.

  • FIG workshop delves into Great Lakes, highlights GNSS techniques

    FIG workshop delves into Great Lakes, highlights GNSS techniques

    Image: FrankRamspott/iStock/Getty Images Plus/Getty Images
    Image: FrankRamspott/iStock/Getty Images Plus/Getty Images

    In one of my previous columns, I described the National Geodetic Survey’s (NGS) plans for replacing the North American Vertical Datum of 1988 (NAVD 88) with the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

    As stated in the NOAA Technical Report NOS NGS 64 Blueprint for the Modernized NSRS, Part 2: Geopotential Coordinates and Geopotential Datum, November 2017, recently revised in February 2021, orthometric heights in NAPGD2022 will be defined through ellipsoid heights and GEOID2022. This means NAPGD2022 orthometric heights will primarily be accessed through GNSS technology.

    Like NAPGD2022, in the next update of the International Great Lakes Datum, denoted as IGLD (2020), the heights in the Great Lakes Region will be developed from GNSS and a gravity model. Unlike NAPGD2022, where users will be estimating GNSS-derived orthometric heights, IGLD (2020) users will be estimating GNSS-derived dynamic heights using GNSS and a gravity model.

    As president of the American Association for Geodetic Surveying (AAGS), I participated in the International Federation of Surveyors (FIG) Virtual Working Week 2021 held June 20–25. For those unfamiliar with AAGS, some activities AAGS pursues are below.

    AAGS Activities

    • Promote a better understanding of geodesy as a science;
    • Create a better appreciation of the value of geodetic surveys and thus encourage greater use of such surveys;
    • Promote geodetic surveys by individuals, government, and private organizations;
    • Foster the adoption of uniform standards and procedures for completing geodetic surveys;
    • Promote the processing, publishing, and disseminating of geodetic survey data and information;
    • Promote programs for testing, calibrating, and evaluating geodetic equipment;
    • Further the development and implementation of the Global Navigation Satellite System (GNSS) for geodetic, land surveying, and land information system applications;
    • Inform the membership of new technical developments by meetings of the association and publications in Surveying and Land Information Science (SaLIS);
    • Promote educational programs in geodesy, geodetic surveying, and related fields;
    • Cooperate with other similar organizations, both national and international, in support of the science of geodesy;
    • Encourage the use of geodetic surveys and mathematical coordinate systems in establishing Public Land Survey System (PLSS) corners

    As stated above, AAGS cooperates with other similar organizations, both national and international, in support of the science of geodesy. AAGS is a voting member of FIG, which means AAGS has the opportunity to nominate and vote for elected officials, and develop policy that is important to all surveyors and mappers.

    On a side note, AAGS is always looking for new members that want to help promote geodetic surveying and related topics. 

    The theme of the FIG Working Week 2021 virtual conference was “Smart Surveyors for Land and Water Management: Challenges in a New Reality.” FIG Commission 5 focuses on meeting the highest level of accuracy for positioning and measurement (see box titled FIG Commission 5). Five 90-minute sessions described some of the efforts of FIG Commission 5.

    FIG Commission 5

    “FIG Commission 5 focuses on meeting the highest level of accuracy for positioning and measurement. It provides the tools, techniques and procedures to educate and train surveying professionals everywhere. Appropriate methodology for data collection and processing are required to be successful in an era of global, integrated geospatial data.”

    These sessions raised surveyor awareness of cutting-edge technology, techniques and procedures for using geodetic data and enhanced global cooperation and standardization in conformance with the ideals expressed by the United Nations resolution for a Global Geodetic Reference Frame.  There were many good papers on positioning and measurement presented at the virtual meeting.  Readers can obtain a list of presentations and papers at this website.

    A paper by Jacob Heck, U.S National Geodetic Survey, and Michael Craymer, Canada Geodetic Survey titled “Updating the International Great Lakes Datum: Enabling the Integration of Water and Land Management in the Great Lakes Region” should be of interest to many U.S. and Canadian surveyors. The box below provides a link to the abstract, paper, handouts and video of the presentation.

    Commission 4 and 5 Joint Session

    Tuesday,
    22 June
    15:00–16:30
    STAGES    		 05.1 – Managing the Land/Water Interface: WGS84 vs. the ITRS
    Commission: 4 and 5
    Chair: Dr. Mohd Razali Mahmud, FIG Commission 4 Chair, Malaysia
    Rapporteur: Dr. Daniel Roman, FIG Commission 5 Chair, United State

    Jacob Heck (U.S.) and Michael Craymer (Canada):

    Updating the International Great Lakes Datum: Enabling the Integration of Water and Land Management in the Great Lakes Region (11046)
    [abstract] [paper] [handouts] [video]

    I encourage everyone to download the paper and obtain an understanding of the future International Great Lakes Datum of 2020.

    The International Great Lakes Datum uses dynamic heights instead of orthometric heights traditionally used for elevations on land.  Figure 4 from Heck and Craymer’s FIG paper, illustrates the difference between orthometric and dynamic heights.  See box titled “Figure 4 from FIG Paper by Heck and Craymer.”  As described by Heck and Craymer, “The dynamic height represents the difference in potential above the reference surface and is the same at all points on a level surface. Orthometric height represents the actual physical distance above the reference surface which may change due to differences in gravity caused by the convergence of equipotential surfaces toward to the poles. Dynamic heights are therefore required for the proper management of water levels and flows in compliance with international regulations and treaties.”

    Figure 4 from FIG paper by Heck and Craymer

    Figure 4. Dynamic heights,HD, and orthometric heights, H. (from FIG 2021 paper by Heck and Craymer)
    Figure 4. Dynamic heights,HD, and orthometric heights, H. (from FIG 2021 paper by Heck and Craymer)

    I would like to highlight, as described in the paper and stated in the summary, that access to the future IGLD will be primarily through GNSS techniques.

    Summary from paper by Heck and Craymer

    The International Great Lakes Datum provides a framework for water level management in the world’s foremost resource of surface freshwater. The current datum, IGLD (1985), is being updated and replaced by IGLD (2020). This updated datum will be fundamentally different in terms of definition and access to the datum. The datum will be identical to the new NAPGD2022 North American geopotential datum and will be compatible with the existing CGVD2013 (if not identical as well) at the reference epoch of 2020. IGLD (2020) is expected to be released in 2025 at about the same time as NAPGD2022. Access to both frames will be primarily through GNSS techniques. This will lead to more consistent heights across the entire Great Lakes region. Further information about the IGLD update can be found on the Coordinating Committee website.

    This new paradigm is important for anyone who works in the Great Lakes region. Actually, it is important to anyone that surveys in the United States, because this new paradigm will also be used to access the North American-Pacific Geopotential Datum of 2022 (NAPGD2022). Anyone following my columns knows this is the future, and that the National Geodetic Survey (NGS) is leading the way in the United States by modernizing the National Spatial Reference System (NSRS).

    Another section that I’d like to highlight is in the box titled “Excerpt from Heck and Craymer Paper on IGLD.”

    Excerpt from Heck and Craymer Paper on IGLD

    For IGLD (2020), the geoid height, N, will be provided by GEOID2022 which will be used to define NAPGD2022 and the expected update to CGVD2013. IGLD (2020) dynamic heights will therefore be equivalent to dynamic heights in NAPGD2022 and CGVD2013 at the 2020 reference epoch. For IGLD (2020) heights of water levels, hydraulic correctors may also need to be applied.

    An important advancement in the development of the new IGLD and North American datums will be the availability of an accurate crustal velocity model that can propagate ellipsoidal heights between different reference epochs. This will enable heights determined at any epoch to be propagated back to the adopted 2020 reference epoch used for IGLD (2020). This will effectively obviate the need to update the entire IGLD datum for the effects of GIA for a much longer period of time, except for incremental improvements to the velocity model and updates to the reference epoch.

    It’s important for users to know that the IGLD (2020) dynamic heights will be equivalent to dynamic heights in NAPGD2022, and an accurate crustal velocity model will be used at any epoch to propagate back to the adopted 2020 reference epoch.  The box titled “Determining Heights in IGLD (2020)” is an excerpt from Heck and Craymer’s FIG paper that describes the process that will be implemented for estimating GNSS-derived dynamic heights in the updated IGLD (2020).

    Determining Heights in IGLD 2020

    In previous realizations of IGLD, spirit leveling was used to determine geopotential numbers which were converted directly to orthometric heights that could then be converted to dynamics heights using equation 4 (𝐻𝐷 =𝐶/𝛾45).

    In the geoid-based IGLD (2020), heights will be primarily determined through GNSS techniques which provide a direct measure of ellipsoidal height. Although spirit leveling is more accurate over shorter distances, GNSS methods combined with an accurate geoid model are capable of providing more accurate heights over moderate to longer distances at a small fraction of the cost of leveling.

    An orthometric height, H, above the geoid is obtained from a GNSS-derived ellipsoidal height, h, above the reference ellipsoid using the geoid height or undulation, N, of the geoid above the reference ellipsoid. This is represented by the simple equation:

    𝐻 = ℎ − 𝑁   (5)

    Using equations (2) – (5), the dynamic height can be obtained from the GNSS-derived ellipsoidal height using:

    𝐻𝐷 =(𝑔̅ ∗ (ℎ − 𝑁))/𝛾45   (6)

    For IGLD (2020), the geoid height, N, will be provided by GEOID2022 which will be used to define NAPGD2022 and the expected update to CGVD2013. IGLD (2020) dynamic heights will therefore be equivalent to dynamic heights in NAPGD2022 and CGVD2013 at the 2020 reference epoch. For IGLD (2020) heights of water levels, hydraulic correctors may also need to be applied.

    An important advancement in the development of the new IGLD and North American datums will be the availability of an accurate crustal velocity model that can propagate ellipsoidal heights between different reference epochs. This will enable heights determined at any epoch to be propagated back to the adopted 2020 reference epoch used for IGLD (2020). This will effectively obviate the need to update the entire IGLD datum for the effects of GIA for a much longer period of time, except for incremental improvements to the velocity model and updates to the reference epoch.

    As stated by Heck and Craymer, hydraulic correctors may also need to be applied to meet IGLD (2020) International policies, procedures and regulations. Information on IGLD (1985) hydraulic correctors can be found on NGS Geodetic Tool Kit Page.

    Another paper presented at FIG Working Week that would be of interest to surveyors is a paper on establishing a geoid-based vertical datum given by Dan Roman, Chief Geodesist at NGS (see the box below). Again, the abstract, paper, handouts and video can be downloaded from the link.

    FIG paper Determining an Optimal Geoid-based Vertical Datum by Dan Roman

    Tuesday,
    22 June
    15:00–16:30
    STAGES    		 05.1 – Managing the Land/Water Interface: WGS84 vs. the ITRS
    Commission: 4 and 5
    Chair: Dr. Mohd Razali Mahmud, FIG Commission 4 Chair, Malaysia
    Rapporteur: Dr. Daniel Roman, FIG Commission 5 Chair, United State

    Roman Daniel (USA):
    Determining an Optimal Geoid-Based Vertical Datum (10876)
    [abstract] [paper] [handouts] [video]

    Roman discusses the concept of establishing an International Height Reference System (IHRS) so all countries could provide physical heights across their boundaries and over the oceans (see the boxes titled “Excerpt from FIG Paper by Dan Roman” and “Summary from FIG Paper by Dan Roman “).  I’ve highlighted several sections that are important to establishing a IHRS.

    Excerpt from FIG Paper by Dan Roman

    2.3 International Height Reference System (IHRS)

    The IHRS is relatively recent compared to the ITRS. Ihde et al. (2017) discussed plans for unification of heights globally, which were updated more recently in Sanchez et al (2021). Just as ITRF realizations are made within the ITRS, there will be IHRF realizations made within the IHRS. The key concept here is that positions will first be realized in the ITRS and then expressed in the IHRS. This means that GNSS-accessed geodetic coordinates will determine your position in a realization of the ITRF. Using those ITRF coordinates, geopotential values will be determined from an equivalent IHRF model based above a datum of W0 = 62,636,853.4 m2 s-2. This effectively gives your position in the Earth’s gravity field, which is a physical height. In adopting such a model then, all countries might provide consistent physical heights across their national boundaries and over the oceans.

    Summary from FIG Paper by Dan Roman

    There is a great deal of activity in modernizing how geospatial data are collected, processed and maintained globally. International agreements are in place to have everyone adopt the Global Geodetic Reference Frame to facilitate geospatial data transfer. The approach will be to realize coordinates in the International Terrestrial Reference Frame and then obtain physical heights from the International Height Reference Frame. Countries may adopt any realization of the ITRF but are restricted to a single geopotential value in the IHRF – W0 = 62,636,853.4 m2 /s2. If comparisons to local tide gauges demonstrate this is not optimum for national definitions of a vertical datum, then an alternate geopotential datum can be determined based on an approach that requires supplemental information.

    GNSS-observations on multiple tide gauges will establish local Mean Sea Level and any variations due to Topography of the Sea Surface. A model of the TSS would be required to remove TSS effects at tide gauges to determine the geodetic coordinates of MSL. Use of a geopotential model enhanced by locally obtained gravity data would yield the geopotential number(s) at tide gauge(s). Assuming multiple tide gauges, then an average or some statistical analysis might be made to determine the optimal geopotential value to select as a geoid.

    NGS’s new modernized NSRS will be compatible with the concept of an International Height Reference Frame.  As stated in Roman’s paper, a recent article by Laura Sanchez, et.al, describes a strategy for the realization of the IHRS (see box below.)

    Excerpt from Strategy for the realisation of the International Height Reference System (IHRS)

    Authors: Laura Sánchez, Jonas Ågren, Jianliang Huang, Yan Ming Wang, Jaakko Mäkinen, Roland Pail, Riccardo Barzaghi, Georgios S. Vergos, Kevin Ahlgren and Qing Liu1

    Abstract

    In 2015, the International Association of Geodesy defined the International Height Reference System (IHRS) as the conventional gravity field-related global height system. The IHRS is a geopotential reference system co-rotating with the Earth.

    Coordinates of points or objects close to or on the Earth’s surface are given by geopotential numbers C(P) referring to an equipotential surface defined by the conventional value W0 = 62,636,853.4 m2 s−2, and geocentric Cartesian coordinates X referring to the International Terrestrial Reference System (ITRS). Current efforts concentrate on an accurate, consistent, and well-defined realisation of the IHRS to provide an international standard for the precise determination of physical coordinates worldwide. Accordingly, this study focuses on the strategy for the realisation of the IHRS; i.e. the establishment of the International Height Reference Frame (IHRF). Four main aspects are considered: (1) methods for the determination of IHRF physical coordinates; (2) standards and conventions needed to ensure consistency between the definition and the realization of the reference system; (3) criteria for the IHRF reference network design and station selection; and (4) operational infrastructure to guarantee a reliable and long-term sustainability of the IHRF. A highlight of this work is the evaluation of different approaches for the determination and accuracy assessment of IHRF coordinates based on the existing resources, namely (1) global gravity models of high resolution, (2) precise regional gravity field modelling, and (3) vertical datum unification of the local height systems into the IHRF. After a detailed discussion of the advantages, current limitations, and possibilities of improvement in the coordinate determination using these options, we define a strategy for the establishment of the IHRF including data requirements, a set of minimum standards/conventions for the determination of potential coordinates, a first IHRF reference network configuration, and a proposal to create a component.

    There’s a very good presentation on the International Height Reference System and International Height Reference Frame (IHRF) given by Laura Sánchez at the “Workshop for the Implementation of the GGRF in Latin America” held in Buenos Aires, Argentina, on Sep 16–20, 2019.

    To support the implementation of IHRF, FIG Commission 5 has a working group that focuses on Vertical Reference Frames. See box below.

    FIG Working Group 5.3

    Vertical Reference Frames

    Policy Issues

      • Educate FIG member agencies on current and future status of regional and global vertical reference frames and height systems
      • Educate FIG member agencies on practical aspects about the implementation of new geopotential datums including:
        • access using geoid height models and a geometric datum
    • redefining heights on existing bench marks to serve as secondary control
    • ties between height systems and local and global mean sea level
    • Develop and expand relationships in IAG Commission 2, UN SCOG, and WG focused on implementing vertical control based on IHRF around the world.
      • IAG will develop an IHRF that will be a component of the UN GGRF.
      •  UN GGRF will encompass both ITRF and IHRF
      • Time varying aspects of the geoid, vertical control and the gravity field must be addressed.

    Chair

    David Avalos-Naranjo, Mexico
    [email protected]

    I have highlighted several statements in the box titled “FIG Working Group 5.3.”  This working group is focused on issues associated with implementing vertical control based on an International Height Reference Frame (IHRF). NGS is working with these groups to ensure that the United States height system will be compatible with the rest of the world.

    I encourage everyone to visit the FIG website and explore the papers given during 2021 FIG Working Week. Here is a list of the FIG Commissions. For more information can be obtained on each commission by clicking on the Commission’s title.

    FIG Commissions

    Commission 1 – Professional Standards and Practice

    Commission 2 – Professional Education

    Commission 3 – Spatial Information Management

    Commission 4 – Hydrography

    Commission 5 – Positioning and Measurement

    Commission 6 – Engineering Surveys

    Commission 7 – Cadastre and Land Management

    Commission 8 – Spatial Planning and Development

    Commission 9 – Valuation and the Management of Real Estate

    Commission 10 – Construction Economics and Management

    Before the American Congress on Surveying and Mapping (ACSM) disbanded, the four-member organization collaborated to convene annual surveying and mapping conferences in the United States. Topics similar to those presented at FIG Working Week were presented at these conferences. I became a member of ACSM in 1972 and learned a lot from attending and participating in these conferences.

    Since these ACSM conferences are no longer being held, I encourage users of geospatial data and GNSS technology to participate in professional societies such as AAGS to enhance their understanding and knowledge of new technical developments in the field of geospatial positioning and measurement. As the current president of AAGS, I am biased, but a benefit of AAGS membership is access to the Surveying and Land Information Science (SaLIS) journal that publishes new technological developments related to geodesy, surveying, and mapping.

  • NGS releases modernized National Spatial Reference System updates

    NGS releases modernized National Spatial Reference System updates

    The National Geodetic Survey (NGS) recently announced two new items related to the modernized National Spatial Reference System (NSRS). First, it announced that there will be a delayed release of the modernized National Spatial Reference System (NSRS). See the box titled “Updates notices from NGS Homepage” for the link to the notice.

    Updates notices from NGS Homepage

    Image: National Geodetic Survey
    Image: National Geodetic Survey

    The box titled “Delayed Release of the Modernized NSRS” provides a summary of the notice. The announcement stated they are performing a thorough review of all tasks and will provide regular updates on their progress. What this means is that the modernized NSRS will not be completed by 2022. Even if it’s delayed a couple of years, it’s never too early to obtain an understanding of the new, modernized NSRS, and start preparing for the transition to the new NSRS.

    Delayed Release of the Modernized NSRS

    (https://www.ngs.noaa.gov/datums/newdatums/delayed-release.shtml)

    NOAA’s National Geodetic Survey (NGS) is announcing a delay in the release of the modernized National Spatial Reference System (NSRS).

    In 2007, NGS began planning for the modernized NSRS, acquiring its first airborne gravimeter, creating and initiating the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project and by 2008 had codified its modernization plans into a Ten Year Plan. At that time, the target completion date was 2018. By 2013, that date seemed unlikely, due to both the broadening of the GRAV-D coverage area and the experience of five years of operational planning and execution.

    In 2013, NGS revised its 2008 Plan, and targeted 2022 as the date of the release of the modernized NSRS. This date was reinforced with a 2018 Strategic Plan revision. By 2017, confidence in hitting the 2022 target was high enough to reach final agreement with Canada and Mexico on a naming convention for certain components, to include “2022” in their names.

    Since 2017, operational, workforce, and other issues have arisen and compounded, causing NGS to recently re-evaluate whether a successful roll-out by 2022 is possible. The most significant impacts have been in workforce hiring and retention, and in meeting GRAV-D data collection milestones, which underpin the NSRS modernization efforts.

    NGS is currently conducting a comprehensive analysis of ongoing projects, programs and resources required to complete NSRS modernization and will continue to provide regular updates on our progress. To get the latest news on NSRS modernization and track our progress, subscribe to NGS News or visit our “New Datums” web pages.

    The second important announcement by NGS was that two Federal Register Notices related to the modernized NSRS were published on July 24. See the box titled “NGS News.”

    Image: National Geodetic Survey
    Image: National Geodetic Survey

    The first Federal notice was titled “Upcoming Changes to the National Spatial Reference System.” See the box titled “Federal Register Notice titled Upcoming Changes to the National Spatial Reference System” for the summary. This announcement provides a statement about the new, modernized NSRS and that it’s going to be published between 2022 and 2025. The information about the modernized NSRS shouldn’t be new to anyone that’s been reading my newsletters, but the Federal Notice makes it official and NGS provides dates of when the modernization will be rolled out.

    Federal Register Notice titled “Upcoming Changes to the National Spatial Reference System”

    (https://www.govinfo.gov/content/pkg/FR-2020-07-24/pdf/2020-16068.pdf)

    Image: Federal Register
    Image: Federal Register

    The second Federal Notice was titled “Consideration of Potential Age Limiting Observations To Be Used To Compute 2020.00 Reference Epoch Coordinates in the National Spatial Reference System.” This is a very important notice that users of NGS published coordinates should read and understand. NGS is considering imposing data age limits that will be part of the new, modernized NSRS. See the box titled “Imposing Age Limits of Data in 2022” for a summary of the Federal Register Notice announcement.

    Imposing Age Limits of Data in 2022

    (https://www.federalregister.gov/d/2020-16084)

    Image: Federal Register
    Image: Federal Register

    My last column highlighted that in the modernized NSRS the only way to get “into the datum” will be through a GNSS survey. It noted that leveling projects generate relative height differences not absolute heights. It emphasized that in the new modernized, time-dependent NSRS, the absolute height will be provided by up-to-date GNSS data; and the relative height differences between leveling marks will be provided by the leveling data. Many of my previous newsletters have explained different aspects of the new NSRS and how it may affect the surveying and mapping community products and services. As the Federal Register Notice implied, at this moment, NGS expects large uncertainties in the vertical component of the Intra-Frame Velocity Model (IFVM) which will translate into the GNSS-derived height Limiting the age of data will help to reduce the amount of uncertainty in the vertical component based on older data. Saying that, this could have an impact on users that rely on coordinates established using data acquired prior to 2010. NGS is requesting that users take new GNSS observations on all stations of interest that haven’t been occupied since the year 2010. The supplementary information in the Federal Register notice contains some very important statements. I have highlighted several statements in the box titled “Supplementary Information from Imposing Age Limits of Data in 2022.”

    NGS hasn’t decided on the date of the age limit but the notice states that “For instance, it is unlikely that such an age-limit will be fewer than 10 years.” This is why NGS recommends the following “that users take new GNSS observations on geodetic control marks of interest that have not been surveyed since January 1, 2010, and asks the users to submit the observations to NGS before December 31, 2021.” Another important item in the supplemental information section is that NGS is enhancing the OPUS-Projects tool to include real-time kinematic and real-time network (RTK/RTN) observations. This should help to facilitate users submitting data on marks of interest so that they will have 2020.0 Reference Epoch Coordinates (REC).

    Supplementary Information from Imposing Age Limits of Data in 2022

    (https://www.federalregister.gov/d/2020-16084)

    SUPPLEMENTARY INFORMATION:
    In 2017, the National Geodetic Survey (NGS) announced its plans to estimate RECs on a five-year cycle in NOAA Technical Report NOS NGS 67, 2019, starting with the first reference epoch at 2020.00, as part of the modernization of the NSRS. In the Technical Report, the exact observations to be used for this estimation were listed as “To Be Determined.” NGS is considering imposing age limits upon the observations that will be used, particularly because of expected uncertainties in the vertical component of the IFVM. These age limits cannot be determined until additional well-structured, data-driven experiments are conducted. Such experiments are expected to occur during the 2020 reference epoch adjustment projects (geometric, orthometric, and gravimetric), which are scheduled for calendar year 2022.

    However, since the cut-off for new observations to enter those adjustment projects is December 31, 2021, any decision to age-limit input observations will come too late for submissions to impact the 2020 RECs. While the cut-off for age-limited observations is unknown, certain assumptions are safe to make. For instance, it is unlikely that such an age-limit will be fewer than 10 years. Older observations may be used in the estimation of 2020 RECs, but this cannot be guaranteed. As such, NGS requests that users take new GNSS observations on geodetic control marks of interest that have not been surveyed since January 1, 2010, and asks the users to submit the observations to NGS before December 31, 2021. Users may either (a) submit existing unsubmitted observations through the OPUS-Share tool or (b) conduct new GNSS observations and submit the data to NGS via the OPUS-Share tool.

    In order to increase the submission of GNSS observations on marks, NGS is prioritizing the finalization of an expanded OPUS-Projects tool, which will allow real-time kinematic and real time network (RTK/RTN) observations to be submitted, rather than the standard four-hour observations required in OPUS-Share. Initial roll-out of this new tool is expected to occur during calendar year 2020.

    This action is designed to increase both the number and the coordinate accuracy of geodetic control points, which in the modernized NSRS will have an estimated 2020.00 REC. Historically, NGS has combined data across multiple decades to estimate geodetic coordinates, yet such efforts have not fully accounted for the lack of information about vertical motion of geodetic control points throughout the years. Since height information is critical to the understanding of floods, failure to compute heights accurately can have negative impacts on property and lives. NGS views periodic re-surveys of geodetic control points, rather than the estimation of coordinates from observations that are years (or even decades) old, as the most effective way to maintain accurate and up-to-date knowledge of geodetic coordinates, including heights. As such, this announcement provides users of the NSRS with advance notice that geodetic control points of interest to them should be re-surveyed for the most accurate representation of geodetic coordinates, including heights.

    NGS has scheduled a webinar for August 27, 2020, to discuss the delayed release of the modernized NSRS. See the box titled “Webinar on Delayed Release of the Modernized NSRS” for the announcement and web link to register for the webinar. I would encourage all users of the NSRS to register for this webinar.

    Webinar on Delayed Release of the Modernized NSRS

    (https://geodesy.noaa.gov/web/science_edu/webinar_series/delayed-release-nsrs.shtml?utm_medium=email&utm_source=GovDelivery)

    Image: National Geodetic Survey
    Image: National Geodetic Survey

    Many users are probably wondering if the delay in the new, modernized NSRS will change the dates of other deadlines. The FAQs webpage addresses some of these questions. I have highlighted a few FAQs in the box titled “Questions from NGS FAQ Website.”

    Questions from NGS FAQ Website

    (https://www.ngs.noaa.gov/datums/newdatums/FAQNewDatums.shtml)

    How will the delay affect the GPS on Benchmarks Phase II deadlines?

    The deadline for submittal of GPSonBM data for the 2022 Transformation tool will remain December 31, 2021

    If SPCS2022 zone designs are completed before other parts of NSRS modernization, will SPCS2022 be released sooner?

    No. SPCS2022 is explicitly defined with respect the four 2022 terrestrial reference frames (not NAD 83), and SPCS2022 will be released along with the roll-out of those frames. If the frames are rolled out prior to other parts of the NSRS modernization, the frames will be accompanied by SPCS2022 (see the previous FAQ about phased roll-outs).
    However, complete definitions of all SPCS2022 zones will be made available as soon as they are finalized. NGS expects that to occur by the end of 2021. Providing zone definitions early will give software vendors, database administrators, and others ample time to adopt and test them in their systems. Doing so will ensure SPCS2022 is available for immediate use upon roll-out of the 2022 terrestrial reference frames.

    My projected height change seems to return me to NGVD 29 heights. Is this a coincidence?
    This is coincidental. It so happens that, in some areas of the country the actual orthometric height in a region happens to be numerically closer to NGVD 29 than NAVD 88. NGVD 29 itself has biases and tilts which make it as inappropriate of an estimate of true orthometric heights as NAVD 88

    [NOTE: I have heard this question from many of my readers so I provided an approximate estimate of the differences between NAPGD2022 orthometric heights and NGVD 29 height values in my June 2017 Survey Scene column. See figure below labeled “Figure 2 from June 2017 Survey Scene Newsletter.”]

    Image: National Geodetic Survey
    Image: National Geodetic Survey

    Figure 2 from June 2017 Survey Scene Newsletter

    Future newsletters will address updates on the modernized NSRS as they become available to the user community.